Mucous membrane secretions are non-homogeneous solutions of long-chain macromolecular substances. The non-homogeneous character of the mucous membrane secretions, which may often be mixed with some very viscous lumps, makes it impossible to make viscosimetric measurements thereon with the usual capillary viscometers. The free flow through the capillaries based on the gravity and the height differential within the vertical capillaries, is interrupted by clogging as soon as viscous lumps of mucus enter the tubes; measurement of the time of throughflow, therefore, becomes impossible.
M. W. Arveson has constructed a capillary viscometer wherein highly viscous fats are forced through a capillary by means of a piston, where the required pressure is measured. However, this apparatus is not suitable for measuring small non-homogeneous quantities of sputum such as those obtained in clinical examinations. The apparatus is, moreover, very expensive to build and extremely unwieldy. Therefore, S. R. Hirsch and R. O. Kory [J. Allergy 39, 265 (1967)] later developed a sputum viscometer wherein the sputum is forced through a perforated hypodermic syringe piston, while the pressure developed thereby is measured. This apparatus gives insufficiently accurate values, however, since no true capillary effect is achieved, and thus laminar flow is not assured. In addition, it is impossible or possible only to a limited extent, to determine different measurement parameters which are important to the evaluation of human sputum or a therapeutic agent which affects it. For a meaningful evaluation of a sputum sample, it is necessary that, first, the overall viscosity of this sputum sample can be measured and, secondly, that the maximum and minimum viscosities of this sputum sample can be determined. It has become evident that conventional viscometers, such as capillary viscometers, falling sphere viscometers and rotation viscometers, are inadequate for determination of these values on non-homogeneous human sputum samples which are solutions of restricted volume and do not follow Newton's law.